/*
 * http://linux.die.net/lkmpg/x569.html
 * http://appusajeev.wordpress.com/2011/06/18/writing-a-linux-character-device-driver/
 */

/*
 *  chardev.c: Creates a read-only char device that says how many times
 *  you've read from the dev file
 */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <asm/uaccess.h>	/* for put_user */

/*  
 *  Prototypes - this would normally go in a .h file
 */
int init_module(void);
void cleanup_module(void);
static int device_open(struct inode *, struct file *);
static int device_release(struct inode *, struct file *);
static ssize_t device_read(struct file *, char *, size_t, loff_t *);
static ssize_t device_write(struct file *, const char *, size_t, loff_t *);

#define SUCCESS 0
#define DEVICE_NAME "cdev_rw"	/* Dev name as it appears in /proc/devices   */
#define BUF_LEN 80		/* Max length of the message from the device */


/* 
 * Global variables are declared as static, so are global within the file. 
 */

static int Major;		/* Major number assigned to our device driver */
static int Device_Open = 0;	/* Is device open?  
				 * Used to prevent multiple access to device */
static char my_data[BUF_LEN];	/* The msg the device will give when asked */
static char *msg_Ptr;


static struct file_operations fops = {
	.read = device_read,
	.write = device_write,
	.open = device_open,
	.release = device_release
};

/*
 * This function is called when the module is loaded
 */
int init_module(void)
{
        Major = register_chrdev(0, DEVICE_NAME, &fops);

	if (Major < 0) {
	  printk(KERN_ALERT "Registering char device failed with %d\n", Major);
	  return Major;
	}

	printk(KERN_INFO "I was assigned major number %d. To talk to\n", Major);
	printk(KERN_INFO "the driver, create a dev file with\n");
	printk(KERN_INFO "'mknod /dev/%s c %d 0'.\n", DEVICE_NAME, Major);
	printk(KERN_INFO "Try various minor numbers. Try to cat and echo to\n");
	printk(KERN_INFO "the device file.\n");
	printk(KERN_INFO "Remove the device file and module when done.\n");
	
	return SUCCESS;
}

/*
 * This function is called when the module is unloaded
 */
void cleanup_module(void)
{
	/* 
	 * Unregister the device 
	 */
	unregister_chrdev(Major, DEVICE_NAME);	
	
	printk(KERN_INFO "Unregistering char device");
}

/*
 * Methods
 */

/* 
 * Called when a process tries to open the device file, like
 * "cat /dev/mycharfile"
 */
static int device_open(struct inode *inode, struct file *file)
{
	if (Device_Open)
		return -EBUSY;

	Device_Open++;
	msg_Ptr = my_data;

	try_module_get(THIS_MODULE);
	printk(KERN_INFO "cdev->device_open");
	return SUCCESS;
}

/* 
 * Called when a process closes the device file.
 */
static int device_release(struct inode *inode, struct file *file)
{
	Device_Open--;		/* We're now ready for our next caller */

	/* 
	 * Decrement the usage count, or else once you opened the file, you'll
	 * never get get rid of the module. 
	 */
	module_put(THIS_MODULE);
	printk(KERN_INFO "cdev->device_release");
	return 0;
}

/* 
 * Called when a process, which already opened the dev file, attempts to
 * read from it.
 */
static ssize_t device_read(struct file *filp,	/* see include/linux/fs.h   */
			   char *buffer,	/* buffer to fill with data */
			   size_t length,	/* length of the buffer     */
			   loff_t * offset)
{
	/*
	 * Number of bytes actually written to the buffer 
	 */
	int bytes_read = 0;

	/*
	 * If we're at the end of the message, 
	 * return 0 signifying end of file 
	 */
	if (*msg_Ptr == 0)
		return 0;

	/* 
	 * Actually put the data into the buffer 
	 */
	while (length && *msg_Ptr) {

		/* 
		 * The buffer is in the user data segment, not the kernel 
		 * segment so "*" assignment won't work.  We have to use 
		 * put_user which copies data from the kernel data segment to
		 * the user data segment. 
		 */
		put_user(*(m